As compared with widely used amorphous silicon (a-Si), an amorphous (noncrystalline) oxide semiconductor has high carrier mobility, a high optical band gap, and film formability at low temperature and, therefore, has been highly expected to be applied for next generation displays which are required to have a large size, high resolution, and high-speed drive, resin substrates which have low heat resistance, and the like.
Of oxide semiconductors, an amorphous oxide semiconductor containing indium, gallium, zinc, and oxygen (In—Ga—Zn—O, hereinafter also referred to as “IGZO”), and an amorphous oxide semiconductor containing indium, zinc, tin, and oxygen (In—Zn—Sn—O, hereinafter also referred to as “IZTO”), which have a considerably high carrier mobility, is preferably used. For example, Non-Patent Documents 1 and 2 disclose a thin film transistor (TFT) including an IGZO thin film of In:Ga:Zn=1.1:1.1:0.9 (atomic ratio) as a semiconductor layer (active layer).
Furthermore, as cases in which an IZTO thin film is used in a semiconductor layer, Patent Documents 1 and 2, and Non-Patent Documents 3 and 4 can be given. Among these, Patent Document 1 discloses an amorphous oxide (IZTO) containing elements such as In, Zn and Sn; and Mo, wherein an atomic composition ratio of Mo to the total number of all metal atoms in the amorphous oxide is 0.1 to 5 atomic %. Furthermore, a TFT including a thin film of IZTO doped with Mo as an active layer is disclosed in the example of Patent Document 1.
In the above technologies, an amorphous oxide semiconductor is used in the semiconductor layer of the TFT, and it is shown that the use of this amorphous oxide semiconductor permits a TFT having a higher mobility than that using a-Si. In contrast, Patent Document 3 is intended to provide a semiconductor layer having higher mobility by crystallizing an oxide semiconductor by high-temperature firing. Specifically, in Patent Document 3, surface roughness of the semiconductor layer causing a problem of characteristic deterioration at the time of high temperature firing is resolved by controlling a firing temperature. More specifically, a polycrystalline oxide semiconductor thin film containing at least one element in the group consisting of In, Ga and Zn is fired at such a temperature range of 660° C. to 840° C. that the thin film is polycrystallized while maintaining a surface roughness Ra of 1.5 nm or less.